DESC:Field of the Invention
The present invention relates to a method of recovering pure 2,3-Dichloropropene.

Background of the Invention
2,3-Dichloroprop-1-ene (DCP) of Formula I is a valuable intermediate in the preparation of pesticides and pharmaceutical products. A number of processes for the preparation of DCP are already known from the literature. One such process is disclosed in EIA-EMP report on Greenfield Multi-purpose plant for manufacturing of various technical grade pesticides and Intermediates, September 2019. The report provides the process of preparation of 2,3-Dichloroprop-1-ene (DCP) from 1,2,3-trichloropropane (TCP) by adding aqueous NaOH solution at room temperature in presence of a phase transfer catalyst. The report provides that for isolation and working-up of DCP, layer separation has to be carried out followed by distillation. The report also mentions that there is a process loss of 264 Kg.

The isolation of 2,3-Dichloroprop-1-ene (DCP) by layer separation followed by distillation provides disadvantages such as substantial decrease in yield of the product and also during phase separation, the DCP comprises at least some of the phase transfer catalysts used for producing the same.

Hence, there is a need in the art to provide an improved process of recovering of DCP which is simple, cost-effective, improves the yield of the product and reduces effluent load.

Summary of the Invention
Accordingly, the present invention provides a method for recovering pure 2,3-Dichloroprop-1-ene (DCP) from a mixture comprising of 2,3-Dichloropropene (DCP) and an aqueous phase, said method comprising the steps of:
(a) heating the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase to obtain a gaseous mixture;
(b) condensing the gaseous mixture so as to obtain a liquid mixture;
(c) obtaining water in the form of a top distillate and pure 2,3-Dichloroprop-1-ene (DCP) in the form of a bottom distillate from the liquid mixture;
(d) mixing the water thus obtained with the mixture comprising 2,3-Dichloropropene (DCP) and the aqueous phase;
(e) continuously performing steps (a) to (d) till all of 2,3-Dichloroprop-1-ene (DCP) has been removed as pure 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase is left.

In an embodiment, the aqueous phase comprises water, Sodium chloride, 1,2,3–Trichloropropane (TCP), Sodium hydroxide and optionally a catalyst. The catalyst is preferably benzyltriethylammonium chloride (TEBA).

In another embodiment the aqueous phase comprises less than or equal to 0.5% of 1,2,3–Trichloropropane (TCP) as determined by gas chromatography.

In one another embodiment, the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase is heated to a temperature in the range of 80oC to 100oC to obtain the gaseous mixture. The gaseous mixture comprises 2,3-Dichloroprop-1-ene (DCP) vapours and steam.

In yet another embodiment the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase is obtained by reacting 1,2,3–Trichloropropane with an aqueous solution of sodium hydroxide.

In an embodiment the reaction between 1,2,3–Trichloropropane (TCP) and aqueous solution of sodium hydroxide is optionally carried out in presence of a catalyst. In another embodiment the catalyst is benzyltriethylammonium chloride (TEBA).

In an embodiment the aqueous solution of sodium hydroxide comprises 5% to 25% of sodium hydroxide in water.

In another embodiment, the reaction between 1,2,3– Trichloropropane (TCP) and aqueous solution of sodium hydroxide is carried out till the TCP in the mixture is less than or equal to 0.5% as determined by gas chromatography.

In one another embodiment, the molar ratio of 1, 2, 3 – Trichloropropane (TCP) and TEBA is in the range from 1: to 0.04 to 0.1.

In yet another embodiment the reaction between 1,2,3–Trichloropropane and aqueous solution of sodium hydroxide is carried out at a temperature in the range of 70 to 80oC for a time period in the range of 2 to 5 hours.

In an embodiment, the aqueous phase as obtained in step(e) is reacted with 1,2,3–Trichloropropane (TCP), sodium hydroxide and optionally TEBA to obtain the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase and performing the steps a) to e).

In another embodiment the solid that sediments from the aqueous phase of step (e) is filtered off.

Detailed description of the invention
The present invention provides a method for recovering pure 2,3-Dichloroprop-1-ene (DCP) from a mixture comprising of 2,3-Dichloroprop-1-ene (DCP) and an aqueous phase, said method comprising the steps of:
(a) heating the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase to obtain a gaseous mixture;
(b) condensing the gaseous mixture so as to obtain a liquid mixture;
(c) obtaining water in the form of a top distillate from the liquid mixture to obtain pure 2,3-Dichloroprop-1-ene (DCP) in the form of a bottom distillate;
(d) mixing the water thus obtained with the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase;
(e) continuously performing steps (a) to (d) till all of 2,3-Dichloroprop-1-ene (DCP) has been removed as pure 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase is left.

In an aspect, the aqueous phase comprises water, Sodium chloride, 1,2,3–Trichloropropane (TCP), Sodium Hydroxide and optionally a catalyst. The catalyst may be any phase transfer catalyst, preferably, TEBA (benzyltriethylammonium chloride). The aqueous phase comprises less than or equal to 0.5% of 1,2,3–Trichloropropane (TCP) as determined by gas chromatography.

In another aspect, the recovery of 2,3-Dichloroprop-1-ene (DCP) from the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and aqueous phase is carried out by heating at a temperature in the range of 80 to 100oC, preferably at a temperature in the range of 85 to 100oC and more preferably at a temperature in the range of 90 to 100oC. A gaseous mixture is thereafter obtained which essentially consists of 2,3-Dichloroprop-1-ene (DCP) vapours and steam.

The method of producing 2,3-dichloro-1-propene (DCP) involves reacting 1,2,3-trichloropropane (TCP) with a base. The reaction to obtain DCP from TCP is carried at a temperature in the range of 70 to 80°C. The reaction time is from 2 to 5 hours and preferably from 2 to 3 hours.

Bases that can be used here comprise organic and inorganic bases. An example that may be mentioned of an organic base is trimethylamine, triethylamine, tributylamine, pyridine, 5-ethyl-2-methylpyridine or quinolone. Examples of inorganic bases that can be used are alkali metal hydroxides. Such as NaOH and KOH, alkaline earth metal hydroxides, such as Ca(OH)2, alkali metal hydrogencarbonates, such as NaHCO3 and KHCO3, and alkali metal carbonates, such as Na2CO3, K2CO3 or Cs2CO3. In a typical method, aqueous solution of sodium hydroxide is used which comprises 5% to 25% of sodium hydroxide in water.

In an aspect, the process for producing 2,3-dichloro 1-propene (DCP) from 1,2,3-trichloropropane (TCP) is carried out in water. Additionally, a phase transfer catalyst is added in order to achieve an adequate reaction rate.

Examples of phase transfer catalysts that can be used are: benzyltriethylammonium bromide and the corresponding chloride, methyltrioctylammonium chloride (trade mark Aliquat 336; present in a mixture with methyl tridecylammonium chloride). It is preferable to use benzyltriethylammonium chloride (TEBA).

In another aspect, the molar ratio of 1, 2, 3 –Trichloropropane (TCP) and TEBA is in the range from 1: to 0.04 to 0.1.
In one another aspect, the reaction between 1,2,3–Trichloropropane (TCP) and aqueous solution of sodium hydroxide is carried out till the TCP in the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase is less than or equal to 0.5% as determined by gas chromatography. The aqueous phase comprises water, Sodium chloride, 1,2,3–Trichloropropane (TCP), Sodium Hydroxide and optionally a catalyst. The aqueous phase may comprise impurities in trace amounts which are formed during the reaction of TCP and aqueous NaOH.

In an aspect, the aqueous phase as obtained in step(e) is reacted with 1,2,3–Trichloropropane (TCP), sodium hydroxide and optionally TEBA to obtain the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase and performing the steps a) to e). The process steps a) to e) can be repeated up to 10 cycles. After few cycles the solids that sediments from the aqueous layer obtained after step e) is filtered off.
The process according to the invention is usually carried out at atmospheric pressure.

The process according to the invention has advantages over the known prior art. The prior art process of recovery of DCP involves: i) layer separation and ii) distillation. The prior art teaches to separate the organic layer (which contains crude DCP) from the aqueous layer by layer separation. The aqueous layer is discarded which increases the effluent load as it contains organic waste. The organic layer is subjected to distillation to recover DCP. The DCP so recovered is of low yield and purity.

The Inventors have surprisingly found a simple one-step process to recover 2,3-Dichloroprop-1-ene (DCP). In the process of the present invention to recover DCP, layer separation is completely avoided. In the process, the mixture of 2,3-Dichloroprop-1-ene (DCP) and aqueous phase is azeotropically distilled which not only leads to simplification of process but also improves the yield by quantitative recovery of the product DCP and thereby the reaction mass is left with zero organic effluent. Further, the aqueous phase obtained after distillation is recycled for the next batch.

The inventors have further surprisingly found that by avoiding the step of layer separation as opposed to the prior art and carrying out the separation of DCP by azeotropic distillation not only simplifies the process but also leads to achieving higher yield.

In an aspect, the method of recovering DCP by the present invention does not involve layer separation of organic layer comprising DCP from aqueous layer at any stage of the process. In another aspect, the mixture of 2,3-Dichloroprop-1-ene (DCP) and aqueous phase which is subjected to distillation should not have been an outcome of layer separation or layer separation followed with mixing of water to the separated organic layer.

The present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

Examples:
Example 1
Preparation of 1,2,3-Trichloro propane
In a 1L four necked bottom flask equipped with mechanical agitator, thermo-pocket, condenser, gas spurge deep tube (circulating 0-5oC solution in condenser) 500 g of allyl chloride was taken. The agitator was started for 5 min. Cooling was applied at around 10 to 15 oC. When the reaction mass attained the temperature of 10oC, chlorine gas was purged for 6-7 hrs at the rate of 92g/hr in the reaction mass until the allyl chloride was less than 0.5%. The purging of chlorine gas was stopped when the reaction mass becomes yellowish in colour. Purge nitrogen until the excess chlorine gas is removed and the reaction mass colour becomes colourless liquid. As such crude will proceed to next step.
Output- 1,2,3-Trichloropropane 947 g (yield 98.13%)
GC Purity – 97.5+0.5

Example 2
Preparation of 2,3- Dichloroprop-1-ene (DCP)
In a 3 L four necked round bottom flask equipped with mechanical agitator, thermo pocket, dean stark condenser (circulating 0-5oC solution in condenser) 5% aqueous solution of sodium hydroxide (65 g) was added under agitation and solution was stirred for 5 to 10 minutes. During stirring, crude 1,2,3-trichloropropane (200 g) was added. Triethylbenzylammonium (TEBA) chloride catalyst (0.7 g) was further added and stirred for 5 to 10 minutes. The reaction mixture was heated to 80°C and temperature was maintained for 2 hrs. After 2hrs the temperature of the reaction mixture was increased to 100°C, during which 2,3-dichloroprop-1-ene and water is azeotropically distilled off in dean stark condenser for 2.5 hrs. Organic layer containing DCP (141 g) was collected as the bottom distillate. The top distillate (aqueous layer) contained distilled water was returned to the round bottom flask. The process continued till all the DCP is collected as the bottom distillate. The aqueous layer was returned to the round bottom flask and to the same NaOH flakes (65g), TEBA and 1,2,3-trichloropropane were added to the reaction mixture at room temperature . The reaction mixture was heated to 80°C. After 2 hours temperature was increased to 100°C, during which 2,3-dichloroprop-1-ene and water is azeotropically distilled off. The organic layer contained DCP was collected as bottom distillate and the aqueous layer is recycled back. This recycling process was carried out 10 times.
144 g of 2,3-dichloroprop-1-ene was obtained, having yield of 96.0% and purity of 98.5% ± 1.5 (by Gas chromatography).

Example 3
Preparation of 2,3-Dichloroprop-1-ene (DCP)
In a 3 L four necked round bottom flask equipped with mechanical agitator, thermo pocket, dean stark condenser (circulating 0-5oC solution in condenser) 5% aqueous solution of sodium hydroxide (65 g) was added under agitation and solution was stirred for 5 to 10 minutes. During stirring, crude 1,2,3-trichloropropane (200 g) was added. Triethylbenzylammonium (TEBA) chloride catalyst (0.7 g) was further added and stirred for 5 to 10 minutes. The reaction mixture was heated to 80°C and temperature was maintained for 2 hrs. After 2hrs the temperature of the reaction mixture was increased to 95°C, during which 2,3-dichloroprop-1-ene and water is azeotropically distilled off in dean stark condenser for 2.5 hrs. Organic layer containing DCP (141 g) was collected as the bottom distillate. The top distillate (aqueous layer) contained distilled water was returned to the round bottom flask. The process continued till all the DCP is collected as the bottom distillate. The aqueous layer was returned to the round bottom flask and to the same NaOH flakes (65g), TEBA and 1,2,3-trichloropropane were added to the reaction mixture at 28°C. The reaction mixture was heated to 80°C. After 2 hours temperature was increased to 95°C, during which 2,3-dichloroprop-1-ene and water is azeotropically distilled off. The organic layer contained DCP was collected as bottom distillate and the aqueous layer is recycled back. This recycling process was carried out 10 times.
141 g of 2,3-dichloroprop-1-ene was obtained, having yield of 94.0% and purity of 98.5% ± 1.5 (by Gas chromatography).

Example 4
Comparative Example (based on prior art process)
Preparation of 2, 3-Dichloroprop-1-ene
A 500 mL three neck round neck RBF equipped with condenser, thermometer pocket, addition funnel and stirring rod with half-moon shape blade was arranged. To this, charged 1,2,3-trichloropropane (100 g, 0.678 mol). To this, slowly added 166.1 g aqueous NaOH solution (0.877 mol) at room temperature followed by addition of TEBA (1.9 g, 0.005 mol). Stirred the reaction mixture at room temperature overnight and raised the temperature to 750C. The reaction progress was monitored by GC. After completion of reaction, water was added (70 g), stirred and settled down reaction mass to separate the layers. Organic layer was distilled to get the title compound at 60-75°C under 30 mmHg vacuum.
Yield = 73.3 % (55.17 g, 0.497 mol);
Purity = 96.36 %. (by Gas Chromatography)
The experimental data obtained in examples 2 and 3 clearly indicate the advantage of
carrying out the separation of DCP by azeotropic distillation and avoiding the step of layer separation, leads to achieving high yield of DCP and high purity.
,CLAIMS:1. A method for recovering pure 2,3-Dichloroprop-1-ene (DCP) from a mixture comprising of 2,3-Dichloroprop-1-ene (DCP) and an aqueous phase, said method comprising the steps of:
(a) heating the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase to obtain a gaseous mixture;
(b) condensing the gaseous mixture so as to obtain a liquid mixture;
(c) obtaining water in the form of a top distillate and pure 2,3-Dichloroprop-1-ene (DCP) in the form of a bottom distillate from the liquid mixture;
(d) mixing the water thus obtained with the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase;
(e) continuously performing steps (a) to (d) till all of 2,3-Dichloroprop-1-ene (DCP) has been removed as pure 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase is left.

2. The method as claimed in claim 1, wherein the aqueous phase comprises water, Sodium chloride, 1,2,3–Trichloropropane (TCP), Sodium Hydroxide and optionally a catalyst.

3. The method as claimed in claim 2, wherein the aqueous phase comprises less than or equal to 0.5% of 1,2,3–Trichloropropane (TCP) as determined by gas chromatography.

4. The method as claimed in claim 1, wherein the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase is heated to a temperature in the range of 80 to 100oC to obtain the gaseous mixture, the gaseous mixture comprising 2,3-Dichloroprop-1-ene (DCP) vapours and steam.

5. The method as claimed in claim 1, wherein the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase is obtained by reacting 1,2,3 – Trichloropropane with an aqueous solution of sodium hydroxide.

6. The method as claimed in claim 5, wherein the reaction between 1,2,3–Trichloropropane (TCP) and aqueous solution of sodium hydroxide is optionally carried out in presence of a catalyst.

7. The method as claimed in claim 2 and 6, wherein the catalyst is benzyltriethylammonium chloride (TEBA).

8. The method as claimed in claim 5, wherein the aqueous solution of sodium hydroxide comprises 5% to 25% of sodium hydroxide in water.

9. The method as claimed in claim 5, wherein the reaction between 1,2,3– Trichloropropane (TCP) and aqueous solution of sodium hydroxide is carried out till the TCP in the mixture is less than or equal to 0.5% as determined by gas chromatography.

10. The method as claimed in claim 5, wherein the molar ratio of 1, 2, 3 – Trichloropropane (TCP) and TEBA is in the range from 1: to 0.04 to 0.1.

11. The method as claimed in claim 5, wherein the reaction between 1,2,3–Trichloropropane (TCP) and aqueous solution of sodium hydroxide is carried out at a temperature in the range of 70 to 80oC for a time period in the range of 2 to 5 hours.

12. The method as claimed in claim 1, comprising reacting the aqueous phase as obtained in step(e) with 1,2,3–Trichloropropane (TCP), sodium hydroxide and optionally TEBA to obtain the mixture comprising 2,3-Dichloroprop-1-ene (DCP) and the aqueous phase and performing the steps a) to e).

13. The method as claimed in claim 1, comprising filtering solid that sediments from the aqueous phase of step (e).